Synthesis and stress analysis of germanium nanocrystals embedded in dielectric matrices 2

In this chapter, the results of an investigation on the effects of annealing temperature, annealing time and Ge concentration on the formation of Ge nanocrystals embedded in HfAlO matrix

Chapter 5

Results & Discussions II:

Synthesis and Growth of Ge

Nanocrystals in High-κ Dielectric

Matrix

5.1 Introduction

Due to the aggressive scaling requirement, Electrically Erasable Programmable Read-Only Memory (EEPROM) is facing the problem of the 10 years charge retention requirement for non-volatility The consequence of this restriction causes the programming and erasing speed of the EEPROM to reach a plateau and prevents the applied gate voltage from being scaled down further To overcome these constraints, the first nanocrystal based flash memory structures

were proposed by Tiwari et al [1], using discrete nanocrystals instead of a

continuous floating gate as charge storage nodes to reduce the local defect related leakage and improve data retention [2]

Although the first nanocrystal based flash memory was demonstrated using Si nanocrystals embedded in silicon oxide matrix [1], several groups have demonstrated a better performance of Ge nanocrystal based memory devices over

Si nanocrystal based devices [3-6] For example, Lu et al have pointed out that

Ge has a narrower bandgap and a similar electron affinity as Si which provides a higher confinement barrier for the retention mode and a smaller barrier for the program and erase modes for the memory devices [6]

Meanwhile, various high dielectric constant (high-κ) materials have also been suggested to replace the gate oxide due to the relentless scaling down of the gate oxide thickness Among which, hafnium oxide (HfO2) based material has emerged as one of the potential candidates because of its reasonably high dielectric constant, low leakage current, comparatively higher crystallization temperature and bandgap [7] There has been a demonstration of very good flash memory characteristics recently based on Ge nanocrystals embedded in hafnium aluminium oxide (HfAlO) matrix [8,9] However, to date, there is a lack of a comprehensive study on the formation and growth of Ge nanocrystals in such a material

In this chapter, the results of an investigation on the effects of annealing temperature, annealing time and Ge concentration on the formation of Ge nanocrystals embedded in HfAlO matrix will be presented Three series of samples with increasing Ge content, namely, Samples A (4 at.%), B (10 at.%) and

C (22 at.%), were prepared for this work We will also present a discussion on the comparison of the growth of Ge nanocrystals in silicon oxide and HfAlO matrices

5.2 Ge nanocrystals in HfAlO matrix

In order to synthesize the Ge nanocrystals in the HfAlO matrix, Samples

A were annealed at 700 to 1000°C for durations varying from 15 to 60 minutes in

N2 ambient The Raman spectra of these annealed samples were featureless indicating the absence of Ge nanocrystals (see Figure 5.1) This implies that there are no Ge nanocrystals in the HfAlO matrix even when the temperature reaches as high as 1000°C Note that, from the RBS results, it was estimated that Samples A contained a relatively low amount of Ge of around 4 at.% in the HfAlO matrix It

is therefore reasonable to expect that the low Ge concentration will increase the barriers to nucleation, thus making it difficult for Ge atoms to nucleate and form nanocrystals As a result, no Ge peak can be observed in the Raman spectra of these samples

Figure 5.1: Raman spectra for Sample A annealed for 15 minutes to 60

minutes from 700 to 1000°C in N2

In order to lower the barriers to nucleation, a series of Sample B was prepared with the higher Ge concentration of about 10 at.% These samples were also annealed at the same conditions as Samples A Figure 5.2 shows the Raman spectra of annealed Samples B It can be seen from this figure that at 700°C, after annealing for 15 minutes, a broad peak can be detected at ~289.4 cm-1 For the same annealing duration of 15 minutes, when the annealing temperature was increased to 800°C, a peak at ~ 299.6 cm-1 with a full width at half maximum (FWHM) value of 8 cm-1 can be observed indicating the presence of Ge nanocrystals in the HfAlO matrix However, at higher annealing temperatures (i.e

900 and 1000°C), no Ge peak can be detected Note that increasing the annealing duration to 60 minutes made no observable changes to all the spectra in Figure 5.2

as compared to those annealed for 15 minutes

Figure 5.2: Raman spectrum for Sample B annealed for 15 minutes to 60

minutes from 700 to 1000°C in N2

Figures 5.3 (a) and (b) are the cross section TEM (XTEM) images of sample B annealed at 800oC for 15 and 60 minutes, respectively Figure 5.3 (a) shows that the nanocrystals are uniformly distributed throughout the matrix and they are approximately ~4 nm in diameter All these nanocrystals show clear lattice fringes (see inset of Figure 5.3 (a)) which indicates good crystallinity of the nanocrystals It is found that a longer annealing duration of 60 minutes enabled the nanocrystal to increase its diameter to ~6.5 nm (see inset of Figure 5.3 (b)) These nanocrystals also exhibited clear lattice fringes and were evenly distributed

in the HfAlO matrix Meanwhile, the diffraction pattern obtained from sample B annealed at 800°C for 15 minutes shown in Figure 5.4 indicates that the HfAlO matrix remain amorphous Note that, a similar pattern was also observed for the sample annealed for 60 minutes

Figure 5.3: TEM micrographs of sample B annealed at 800°C for (a) 15

minutes and (b) 60 minutes The insets are the HRTEM micrographs of the nanocrystals from the corresponding samples

Figure 5.4: The diffraction pattern for sample B annealed at 800°C for 15

minutes

According to simulation results of She et al [10], for FLASH applications,

the optimum size of Ge nanocrystals embedded in SiO2 should be 5 nm in diameter for the most efficient write/erase speed and retention time For the case

of Ge nanocrystals embedded in HfAlO, the optimum size should also be similar

or slightly larger as the barrier height is lower [11] Thus the present way to synthesize Ge nanocrystals provides a promising way of achieving the ideal size for Ge nanocrystal FLASH memory devices The size required could be obtained

by controlling the annealing duration, i.e if a larger size is desired, the annealing time could be increased

The XTEM image of sample B annealed at 900°C for 15 minutes (see Fig 5.5 (a)) reveals that there is an absence of Ge nanocrystals in the HfAlO film This is in good agreement with the featureless Raman spectrum of this sample shown in Figure 5.2 Figure 5.5 (b) shows the diffraction pattern of this sample annealed at 900°C for 15 minutes It shows that the HfAlO film had crystallized when annealed under this condition The crystallization of HfAlO at 900°C has

also been reported by Teresawa et al [7] The crystallized HfAlO structure could

enhance the diffusion of Ge atoms through grain boundaries [12]and lead to a reduction in Ge concentration in the HfAlO film This would raise the barriers to nucleation Furthermore, a higher annealing temperature will lead to an increase

in the critical nucleus size and would also raise the barriers to nucleation

Figure 5.5: (a) Cross section transmission electron microscopy (TEM) image

of sample B annealed at 900°C 15 minutes anneal, (b) the corresponding diffraction pattern of the annealed sample

Figure 5.6 (b) shows the SIMS results of the as-sputtered sample B and that annealed at 900°C for 15 minutes It was found that there is indeed significant diffusion of the Ge atoms away from the bulk of the HfAlO film either into the ambient or into the Si substrate As a result, it is reasonable to expect no Ge nanocrystals in samples annealed at 900°C One would also expect the reduction

of Ge in the HfAlO matrix to increase further for a longer annealing duration This will definitely prevent the formation of Ge nanocrystals in the HfAlO matrix,

as confirmed by the featureless Raman spectrum of such sample in Figure 5.2

At a higher annealing temperature of 1000°C, firstly, the diffusivity of the

Ge atoms would be even higher than that at 900°C as the Ge atoms would be in its liquid state and, secondly, the HfAlO matrix would crystallize even more These

two factors will cause a significant depletion of Ge in the HfAlO matrix, thus make it impossible for the nucleation and growth of the nanocrystals Again, this

is confirmed by the Raman spectra of samples annealed at 1000°C for 15 and 60 minutes shown in Figure 5.2

Figure 5.6: (a) Secondary ion mass spectrometry (SIMS) profiles of

as-sputtered and annealed (800°C for 15 and 60 minutes) Samples B, (b) SIMS profiles of as-sputtered and annealed (900°C for 15 minutes) Samples B

In comparison, Figure 5.6 (a) shows the SIMS results of the as-sputtered sample B and those annealed at 800°C for 15 and 60 minutes The Ge concentration profiles show that there is virtually no change in the Ge concentration in the bulk of the HfAlO matrix for the as-prepared and the annealed samples There is a slight reduction in the Ge concentration at the surface of the annealed samples probably due to outdiffusion of Ge when annealed at elevated temperatures There is also some indication of Ge diffusion

to the Si substrate when annealed as there is an increase in the Ge concentration near the HfAlO-Si interface for the annealed samples On the whole, Fig 5.6 (a)

indicates a good retention of Ge in the bulk of the HfAlO film when annealed at 800°C This provides the Ge supersaturation and driving force for the Ge nanocrystal formation and growth

5.3 Ge nanocrystals in crystallized HfAlO matrix

It has been reported that the existence of Ge and Ge diffusion may decrease the crystallization temperature of HfAlO [13] In order to examine the effects of matrix crystallization and Ge concentration on the formation of Ge nanocrystals, a series Sample C (Ge content 23 at.%) were prepared Figure 5.7 shows the Raman spectra of Samples C annealed at 700°C and 800°C for 15 minutes and 60 minutes Similar to samples B, samples C also show a broad amorphous Ge band when annealed at 700°C for 15 and 60 minutes However, a sharp Ge peak located at ~300 cm-1 with a FWHM of 8 cm-1 is observed for Sample C annealed at 800°C for 15 minutes Further increase in the annealing duration to 60 minutes resulted in the disappearance of such peak It should be pointed out here that annealing Sample C at 900°C and beyond resulted in severe bubbling and evaporation of the HfAlO film This is probably due to the fact that HfAlO film with such a high Ge content will significantly deteriorate the film quality and thus reduce its thermal stability

Figure 5.7: Raman spectrum for Sample C annealed at 700 and 800°C in N2

for 15 and 60 minutes

Figures 5.8 (a) and (b) show the XTEM and HRTEM images of a Sample

C annealed at 800°C for 15 minutes, respectively Numerous Ge nanocrystals can

be observed in Figure 5.8 (a) and this agrees with the clear Raman peak observed

in Figure 5.7 Note that from the energy dispersive X-ray (EDX) analysis, the Ge content at the bulk of the films and at the HfAlO/Si interface was estimated to be 11.8 and 17.6 %, respectively However, unlike Sample B, it is found that the HfAlO film started to crystallize at 800°C for Sample C, as can be clearly seen from the lattice fringes found in the HfAlO film in Figure 5.8 (b) This is further proven by the diffraction pattern of the sample (see the inset of Figure 5.8 (a)) In addition, X-ray diffraction (XRD) spectrum of the sample shown in Figure 5.9 clearly confirms that there is indeed Ge reflection peak and that the matrix has

crystallized with the occurrence of the HfO2 (211), (400), (402) and (611) reflection peaks [13] It should be noted that, for sample B annealed at the same condition, the HfAlO matrix remained amorphous with a weak HfO2 halo at ~35°

and the Ge (111) and (220) reflections [14] Liu et al have also reported a

decrease in the crystallization temperature for HfAlO films with the incorporation

of Ge They observed localized crystallization of the HfAlO plus Ge films at 800°C [15] This is in good agreement with our results shown in Figure 5.8 (b) In fact, by comparing the results of Figure 5.3 (a) and 5.8 (a), we show that the incorporation of Ge into HfAlO matrix will lower the crystallization temperature

of the HfAlO film from 900 to 800°C when the Ge concentration in the matrix reaches a value of ~ 23.3 at.%

Figure 5.8: (a) XTEM and (b) high magnification XTEM of Sample C

annealed for 15 minutes at 800°C in N2 The inset is the diffraction pattern of the sample in (a) showing that it has crystallized

Figure 5.9: θ-2θ x-ray diffraction patterns of the samples annealed at different

between the film and the surroundings (c.f Sample B); that provides a larger driving force for Ge to diffuse away from the matrix [16,17]

Figure 5.10: SIMS depth profile of Ge concentration in the HfAlO matrix for

as-sputtered sample and sample annealed for 15 minutes at 800°C

in Sample C

It should be pointed out that, although the crystallization of the HfAlO matrix of Sample C makes it difficult for the nucleation of the nanocrystals as more Ge atoms would be able to diffuse out of the matrix via the grain boundaries, there is still ~11.5 at.% of Ge in the HfAlO matrix From the results

of Sample B, a Ge concentration of ~10.5 at.% would be sufficient for the formation of nanocrystals when annealed at 800°C for 15 minutes Therefore, a

critical Ge concentration (at a particular annealing temperature) is the most important factor that determines the formation of the nanocrystals

It has been also observed from the TEM image of a Sample C annealed at 800°C for 60 minutes (see Figure 5.11) that there was a reduction in the number

of Ge nanocrystals in the bulk of the HfAlO film when the annealing time was increased from 15 to 60 minutes The EDX analysis showed that there was a significant decrease in the Ge concentration in the bulk of the matrix to 4.5 at.% The Ge concentration near the HfAlO/Si interface was estimated to be 20.3 at.%

Figure 5.11: Cross section transmission electron microscopy (TEM) image of

sample C annealed at 800oC for 60 minutes

It appears that with an increase in annealing time, the Ge atoms in the matrix had out-diffused to the ambient instead of aiding in the growth of the existing nucleus, which is probably due to the crystallization of the matrix The

Ge content near the HfAlO/Si interface was kept at a relatively high level of

~20.3 at.% because this interface was a very efficient sink for the Ge atoms in the HfAlO matrix Note that such a phenomenon has also been observed in the SiO2 +

Ge on Si system [18,19] The higher Ge concentration at the HfAlO/Si interface will lead to the growth of large Ge nanoclusters Such large nanoclusters have a much lower equilibrium concentration of dissolved Ge at their interface than the smaller nanocrystals in the bulk of the matrix Due to the different Ge gradient between the smaller nanocrystals and the bigger nanoclusters, it is possible for the nanocrystals to dissolve and provides the Ge atoms for the growth of the nanoclusters

5.4 Comparison of nanocrystals growth in HfAlO and Si oxide matrix

It has been shown in Figure 4.6 of the pervious chapter that a SiO2 + Ge system with medium Ge concentration of 9.9 at.% (from RBS experiments) shows the presence of a clear crystalline Ge Raman peak at annealing temperatures ranging from 700 to 1000°C after annealing for 15 minutes Figure 4.8 reveals that a 15 minutes annealing at 800°C resulted in the formation of nanocrystals of around 8 nm in diameter These nanocrystals are also uniformly distributed throughout the matrix but they are relatively much denser and more numerous as comparing to the HfAlO + Ge system with similar concentration and the sample annealing condition (see Figure 5.3(a))

The faster nucleation and growth rate of these nanocrystals in SiO2 matrix

as compared to those synthesized in HfAlO under similar conditions implies that